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Re: Stream-based Scanning of File-Buffers
From: |
Pascal J. Bourguignon |
Subject: |
Re: Stream-based Scanning of File-Buffers |
Date: |
Fri, 25 Sep 2009 15:16:31 +0200 |
User-agent: |
Gnus/5.101 (Gnus v5.10.10) Emacs/22.2 (gnu/linux) |
Nordlöw <per.nordlow@gmail.com> writes:
> Is there a way to perform string/regexp scanning (using search-forward
> or re-search-forward) in a file-buffer whose contents is loaded only
> when it's needed, kind of like file streams in C, so that I only do a
> physical read on the blocks that I actually scan and then skip the
> rest of the file as soon as I get a hit.
There are emacs lisp primitives that allow you to load a file on
demand. However, the regular expressions functions will only work on
a string or a buffer (faster on a buffer), so when you reach the end
of the buffer without a match, you will have to further load a chunk
and retry the regexp. In general, for a regexp such as "a.*z", if
your file contains a 'a' in the first position, a lot of 'b's, and a
'z' in last position, you will have to load the whole file to match it
with the provided regexp functions.
Of course, you may write your own regexp compiler. A regexp such as
"a.*z" would be compiled to something like:
(lambda (stream)
(loop
for ch = (read-char stream nil)
while (and ch (char/= ch ?a))
finally (return (if ch
;; got a 'a'
(loop for ch = (read-char stream nil)
while (and ch (char/= ch ?z))
finally (return (if ch
'found-match
nil)))
;; got eof
nil))))
In such a case, you need only a one character buffer: ch.
Other regular expressions may need bigger buffers.
--
__Pascal Bourguignon__